There's really not much to demonstrate.
1. Create a SHFB Project and add the assemblies common to the other projects to it as documentation sources. Remove them as documentation sources from the other projects.
2. In each of the other projects, add the Additional Reference Links plug-in to the Plug-In Configurations.
3. Configure the plug-in by adding the project created in step 1 with an appropriate link type. For MS Help Viewer, that would be Id.
4. Build all three projects and install the content in MS Help Viewer.
As I said, if you still have problems,send me a small example that demonstrates the issue. Otherwise, I'm just guessing at what the problem is.